Tire sidewalls are often coated with protective paints or coatings, particularly tire sidewalls having a color which contrasts with the typical black-colored remainder of the tire. The general purpose of such a protective coating is to inhibit or prevent migration of chemicals to the tire rubber composition from the surface of an adjacent black-colored tire which would otherwise tend to stain and, thus, discolor the rubber. Staining caused by migration of chemicals upon contact from an adjacent tire usually becomes evident after exposure of the contacted rubber surface to ultraviolet light such as sunlight. Such contact staining phenomenon is well known.
Rubber tires are typically black in color due to the carbon black reinforcement contained therein. If a contrasting or other color is desired for a part of the tire, then a coloring pigment is used rather than the carbon black. Conventionally, various of the compounding ingredients used in the formulation of rubber for the manufacture of rubber products such as tires are of the staining type; namely, amine-based antidegradants and aromatic rubber processing oils. These chemicals tend to migrate to the surface of the rubber and may stain or discolor the rubber surface of another article which contacts it for a period of time, particularly after the contacted rubber is exposed to ultraviolet light.
Often, a polyvinyl alcohol (PVA) based coating is used to protect the contrastingly colored rubber surfaces from other rubber surfaces of the staining type because the polyvinyl alcohol itself tends to be a barrier for the migration of such staining material. Moreover, the PVA can normally be easily removed from the coated rubber surface, when and if desired, by simple water-washing, usually with a soap solution, since the PVA itself normally has a relatively high degree of water solubility, particularly when the water contains a surfactant. Thus, the PVA coat is typically used for storage and shipping conditions and is usually removed by water-washing by the user. The PVA coating is normally used in conjunction with (mixed with) a plasticizer so that the resulting PVA-based coating has a degree of flexibility.
Tires are often stored under varying circumstances for various purposes so that they may be in contact with each other for a period of time such as, for example, storage during their shipping or transportation to various locations and warehouse storing. The storage conditions can vary considerably and, sometimes, the tires are stored under relatively high humidity conditions and sometimes at elevated temperatures.
Under such circumstances, where the tire surface has a PVA-based coating thereon, it has been observed that under relatively high humidity conditions the PVA-based coatings soften and easily scuff or abrades away, particularly as one tire may rub against another during shipment, thereby leaving a portion of the contrastingly colored sidewall unprotected and susceptible to chemical contact migrating staining from other adjacent and contacting rubber products, including tires. At elevated temperatures, the aforesaid staining ingredients have a greater tendency to migrate to the surface of its host cured rubber product and, consequently, contribute to the staining of an adjacent and contacting rubber surface which is contrastingly light colored--even through many conventional protective barrier coatings (including PVA-based coatings).
Attempts have been made to provide a coating composition and a resultant coated rubber product, particularly a coated tire sidewall rubber surface, which will retard, inhibit and/or prevent the migration of amine-based antidegradants and aromatic rubber processing oils onto the rubber sidewall surfaces, such as a white sidewall, from an adjacent and contacting black rubber surface. For example, U.S. Pat. Nos. 5,137,070 and 5,240,056 to Kansupada et al describe protective coatings for the white sidewalls on tires based on polyvinyl alcohol, and including a monovalent water soluble salt of methylvinylether/maleic acid copolymer and a plasticizer. U.S. Pat. No. 4,911,218 to Patitsas describes a coating prepared from polyvinyl alcohol and mica or glass, and U.S. Pat. No. 5,149,591 to Patitsas et al describes a coating prepared from a blend of polyvinyl alcohol and water-dispersed polyurethane.